Influence Of Beam Directivity And Focal Depth In The Femtosecond Laser Micromachining

With the increase of the national car ownership,environmental pollution brought by the automobile is more and more serious,China put forward China’s stage 6vehicle emission standards.In order to improve fuel combustion efficiency,reduce automobile exhaust emission and meet the exhaust emission standard,the manufacturing requirements of engine key components in automobile field are becoming increasingly stringent.As one of the key components in the engine,the nozzle is required to have a smaller diameter,a higher depth-diameter ratio and a higher machining quality.Traditional machining methods,such as mechanical machining,Electrical Discharge Machining,Electrochemical Machining,are difficult to meet the requirements of high quality micro-circular hole fine machining.Femtosecond laser has many advantages,such as ultra-high peak power,ultra short pulse width,small thermal effect and other processing methods,which can not be achieved by other processing methods.Therefore,femtosecond laser ultra-fine processing technology has become an effective solution to realize ultra-fine fine circular hole processing,and has attracted wide attention.In this paper,the interaction mechanism between femtosecond laser and metal materials is explored,and the removal principle of multi-pulse femtosecond laser metal materials is systematically explored based on the theory of two-temperature model,which provides a theoretical basis for femtosecond laser micro-hole processing.At present,there are still a series of problems in femtosecond laser ultra-fine circular hole processing technology.This paper analyzes and explores new methods to improve the ability of femtosecond laser ultra-fine circular hole processing from two aspects: beam stability and large depth-diameter ratio.The specific work is as follows:1.For the problem of beam stability,the influence factors of beam stability are analyzed.With the rapid development of numerically controlled machine tools,measurement sensors,beam control system and other aspects,the influence of beam pointing stability gradually highlights.In this paper,the beam pointing stability is taken as the breakthrough point to improve the processing ability,the influence of the beam pointing stability on the processing is analyzed,and the method of improving the beam pointing stability is studied.The external beam pointing stability system composed of orientation sensor and fast rotating mirror is used to improve the beampointing stability,and the micro circular hole processing experiment is carried out.With the improvement of beam pointing stability,the consistency of micro hole is improved,and the processing quality of single micro hole is improved.2.For the problem of large aspect ratio,the focal depth area when femtosecond laser is focused is used as the material processing area.For the two-dimensional galvanometer processing system,the depth diameter ratio of micro hole processing can only reach 3:1,and the depth diameter ratio of micro hole processing is limited by the focal depth.In this paper,the long focal depth beam and focus axial control are studied.The beam shaping method and optical elements for beam shaping are analyzed.A method based on Fresnel lens wavefront phase distribution function and phase superposition principle is proposed to realize long focal depth beam shaping.Long focal depth beams with different focal depth stretching times(2 times,4 times,10 times)are realized.At the same time,the axial control of focus is realized by using Fresnel lens wavefront phase distribution function.A new method based on high damage threshold spatial light modulator and two-dimensional galvanometer processing system is proposed,and the optical system is built for processing experiments.For the processing of long focal depth beam micro hole,the edge side lobe at the focal spot will reduce the machining quality of the micro hole.At the same time,due to the energy dispersion,the larger aspect ratio micro hole processing is not used.The long focal depth beam processing experiment shows that for the non transparent material processing,the long focal depth beam is difficult to achieve the desired processing effect.Based on the real-time modulation characteristics of the liquid crystal spatial light modulator,the focus axial control is realized in the processing process.The focus is controlled to move along with the increase of machining depth until the micro hole machining is completed.The diameter of the stainless steel sample with the thickness of 2mm is realized on the premise of ensuring high machining quality.The new method uses galvo scanning laser drilling systems and spatial light modulator to process microhole.,which has a good application prospect in the field of laser processing.